1. Field of the Invention
This invention relates to the field of method and apparatus for executing seeks in a disk drive environment.
2. Background Art
Mass storage for computer systems is typically provided by a magnetic or optical media storage system, such as rigid or flexible disk storage systems. In a magnetic system, a rotating disk having a magnetic media layer on the surface is accessed by a "read/write" head which is used to store and retrieve information from the disk surface. To store information on a magnetic media disk, flux reversals are induced in the magnetic particles comprising the disk's surface. When a magnetic read/write head is passed over the flux reversals, a signal is induced in the head which can be decoded to convey information stored on the disk.
In a preformatted or write once, optical disk drive, data is stored as a series of pits arranged in concentric or spiral tracks on a disk surface. The read/write head is replaced by a lens assembly which is used to project a light beam, (such as a laser beam), onto the disk surface. The light beam is modulated by the pits in the disk and the modulated light beam is reflected from the disk to an optical pick up device which can produce an output signal dependent on the modulation of the light beam. In a magneto-optical disk drive, magnetic domains are oriented so that the polarization of a read light beam is modulated and this modulated beam is detected. In an optical disk drive, a movable lens assembly is mounted on a relatively large base carriage. The base carriage is moved back and forth in a radial direction relative to the optical disk. The lens assembly moves radially relative to the disk and relative to the base carriage.
In the present application, "read/write" head will be used interchangeably with the magnetic head of the magnetic medium drive or the lens assembly of the optical drive. A read/write head refers to any suitable means for accessing or providing data to a storage medium.
In the prior art, data is arranged on a disk in a series of concentric "tracks". For read/write operations, a movable read/write head is selectably positioned over one of the tracks and used to write or retrieve data from the disk surface. Each of the tracks is divided into sectors. Each sector of each track of the disk is a memory address location which can be accessed by control software for data storage and retrieval. When the disk drive system receives a request for information, or when it is writing information to a particular address, the read/write head must be moved from its present location to the desired track and sector location. The movement of the read/write head from one position to a destination position is known as a "seek". Because information is written to and from the disk surface in a random fashion, it is generally necessary to perform a number of seeks during each disk access. Seek operations take up the majority of the execution time of a disk access operation. Therefore, drive performance is directly dependent on seek efficiency.
In the prior art, a seek is executed in two stages. The first "coarse positioning" stage is used to move the read/write head into the general destination area of the seek. At the completion of the first coarse stage, a second "fine positioning" stage is implemented to position the read/write head directly over the desired track. A disadvantage of this prior art system is that two seek stages are required. This adds to the time of the seek, reducing drive efficiency. Further, the first stage may result in the read/write head being positioned as many as a hundred tracks away from the desired destination track. Because the fine positioning stage requires small steps to be accurate, the length of the fine positioning stage is increased.
In a prior art seek method for an optical drive, the base carriage is accelerated at the beginning of the seek for coarse positioning. As the base carriage moves the lens assembly across the disk, the lens assembly is utilized to count track crossings on the disk or on a base mounted reference scale. When the count indicates that the appropriate track location is reached, the base carriage is stopped and track following circuitry is utilized to position the lens assembly to lock onto the desired track. Further, by relying on the disk surface to provide position information during the seek (by counting track crossings) the prior art seek method is sensitive to flaws in the disk media. Such flaws may indicate a track crossing where none in fact exists resulting in incorrect positioning of the lens assembly.
Therefore, it is an object of the present invention to provide a seek method in which a seek can be executed in a single stage.
It is another object of the present invention to provide a seek method in which a single control loop executes the seek operation.
It is yet another object of the present invention to provide a seek method in which track following circuitry controls the seek.
It is still another object of the present invention to provide a seek method which is independent of media defects.